1. Field of the Invention
This invention relates to a network device of a transfer switching type and a frame transfer method.
2. Description of the Related Art
Real-time communication technology such as Institute of Electrical and Electronic Engineers (IEEE) 1394 employs a transfer system (referred to hereinbelow as “cyclic transfer”) using a cycle including real time data and best effort data. FIG. 5 shows a standard cycle pattern. As shown in FIG. 5, cycles are repeated by taking predetermined 125 μsec as one cycle. Packet data, that is, a frame, occupying a predetermined time band within this one cycle is transferred between network devices. Here, the first half of one cycle is taken as a reserved transfer interval and the second half is taken as a free transfer interval.
The reserved transfer interval is used for real time data communication. In this interval, for example, as shown in FIG. 5, a predetermined time, that is, bands 1 to 5 are reserved for frame transmission. The reserved bands 1 to 5 are used only between the respective set devices. Where frames A1 to A5 of real time data are arranged in the reserved bands 1 to 5, a constant amount of data communication is possible within a constant time. By contrast, the free transfer interval is used for best effort data communication that has no real time property. In this interval, no band is reserved. For example, as shown in FIG. 5, where a band 6 of this interval is vacant during data transfer, a frame B1 is arranged therein and data communication between the devices is performed. Frames B2 to B5 are similarly arranged.
For example, a daisy-chain connection composed of network devices 11 to 14 shown in FIG. 6 and a star connection composed of network devices 11, 12, 13, and 15 can be considered as a network configuration that realizes a cyclic transfer. Each network device has a bridge function, and network devices 12, 13, and 15 can transfer a frame transmitted from a network device on one side of the device to a network device on the other side. As a result, communication can be performed by using a bridge function even between the network devices that are not directly connected to each other.
There is a trend to applying the above-described cyclic transfer to Ethernet (registered trademark), which is a Local Area Network (LAN) standard, and high speed and high reliability of data communication with the cyclic transfer are sought for a LAN using the Ethernet (registered trademark).
A cut-through system and a store-and-forward system are available as frame transfer systems. In the cut-through system, the transmitted data are immediately transferred without error checking. As a result, the delay time is small. However, data having errors can fill the network and transmission efficiency can drop. In the store-and-forward system, the transmitted data are transferred after accumulation. Therefore, errors can be checked. However, the delay time increases over that in the cut-through system by the time required for data accumulation.
Thus, the cut-through system and store-and-forward system have exactly opposite features. Accordingly, the cut-through system and store-and-forward system are sometimes used together so that switching therebetween is possible. For example, Japanese Patent Application Publication No. 9-18507 (JP-A-9-18507) and Japanese Patent Application Publication No. 9-149068 (JP-A-9-149068) disclose a method for switching from the cut-through system to the store-and-forward system when the number of errors or error frequency in data rises.
However, when the method disclosed in JP-A-9-18507 or JP-A-9-149068 is applied to the cyclic transfer, a delay can occur and there is a risk of data being not present in the reserved band. More specifically, where a network device that performs transferring is switched to the store-and-forward system due to error occurrence, transfer delay is caused by data accumulation. As a result, data are not present in the band reserved by the network device that is a transmission source, and a shift can occur in a band reserved by another network device. In this case, one of competing data has to wait, thereby further increasing the delay.